This is typically an issue with the payment gateway and/or processor rather than the site or the platform underneath it. You probably had a better error message in the second experience because the site uses a different payment gateway/processor. That's not to say the first site's experience is great, but payment processor selection has lots of competing considerations... processing fees, ability to support enterprise-grade MSA/SLA requirements, security, audit trail, etc etc. (Others are also correct in observing that more vague error messages are actually a strategy by some processors to avoid tipping off fraudsters.)